This invention relates to a cable for connection to sensors permanently downhole within a well, to a method of placing such a cable downhole, and to a well with such a cable permanently in position.
Cables used within wells to provide power downhole are typically circular in cross-section, although it is known to use power cables with a non-circular cross-section downhole. These power cables are placed within production tubing to reach, for example, a motor or pump and are large gauge insulated copper conductors bound together with a pre-formed/interlocking steel tape. The power cable is not placed permanently downhole, generally being replaced when the motor or pump to which it supplies power is removed from the well for repair or maintenance.
It is an aim of the present invention to provide an improved cable for use downhole.
In accordance with one aspect of the present invention, there is provided a cable for connection to sensors permanently downhole, the cable comprising a plurality of elongate conductors capable of operative connection to sensors, a sheath surrounding the elongate conductors and holding the conductors so as to extend substantially parallel to an elongate axis, wherein the sheath has a cross-section, perpendicular to the direction of the elongate axis, which has a major dimension and a minor dimension. The cable thus has a substantially elongate, or flattened, cross-sectional shape.
Such a cable is particularly advantageous in permanent monitoring of wells producing oil where sensing of parameters downhole is required throughout the life of a well.
The cross-section of the sheath may be substantially in the shape of an ellipse, which simplifies manufacture of the cable. However other types of cross-section are also suitable, and thus the sheath may have a cross-section where the major dimension and minor dimension are provided by a shape comprising a circle with wing-like portions attached on opposite sides of the circle. Alternatively the cross-section may be substantially in the shape of a crescent.
The sheath preferably comprises a resilient material, so as to provide a robust outer surface of the cable which prevents the cable breaking when being installed downhole.
The resilient material may be a thermoset material, such as nitrile rubber, to allow for ease of welding of electrodes to the cable.
The conductors are preferably each made from a solid conductive material, such as copper, so as to provide maximum conductivity in minimum cross-sectional area. As the cable is intended primarily for use downhole, the conductors may desirably be plated with a protective material, such as nickel, to provide protection against corrosive liquids and gases. Additionally, the conductors may also include optical fibres.
Preferably each conductor is insulated with a polymer material, such as ethylene propylene copolymer, so as to electrically isolate the conductor from other conductors carried within the sheath.
Further reduction in electrical interaction between the conductors may be achieved by a number of elongate conductors being grouped together. Each group has the conductors inter-weaved in a helical arrangement so as to reduce electrical cross-talk amongst the different conductors within the group.
Typically the cable will include four groups of conductors, each group consisting of four conductors. However the number of groups used, and the number of conductors in those groups, will depend on the number of conductors used in the cable. The cable may also comprise a plurality of strengthening elements, spaced from the conductors, so as to improve robustness and rigidity of the cable. Typically each strengthening element is a wire cord or rope of greater diameter than each group of conductors, and generally a first wire rope is placed near one end of the major dimension, and a second wire rope placed near the opposite end of the major dimension. The wire ropes provide crush resistance should the cable be subjected to force perpendicular to its elongate axis, and also stiffen the cable and provide axial strength. Cable stiffness is of particular advantage when feeding the cable downhole and cementing the cable in place.
The wire cord may comprise a number of separate strands and may be hollow to allow passage of a fibre optic cable within the wire cord. This is of particular use where optical signals are to be transmitted along the length of the borehole as the hollow wire cord provides both a conduit for the fibre optic cable and also a protective shield for the fibre optic cable.
The invention also lies in a method of cementing a well, comprising forming a borehole, placing elongate tubing within the borehole to form an annulus in the borehole, and placing within the annulus a cable with a cross-section which has a major and a minor dimension, such that the minor dimension extends along a radius of the borehole, and passing cement, or thixotropic fluid, downhole to secure the cable in the annulus.
The cable may have the preferred features as set out above.
The cable preferably adjoins the elongate tubing, such that the major dimension of the cross-section extends generally in an arc within the annulus.
As the minor dimension of the cross-section runs partially along a radius of the borehole, the distance from an outside wall of the borehole to the cable is maximised. This reduces the likelihood of mud not being displaced from the region between the cable and the outer wall of the borehole when cementing occurs.
The method may further comprise securing the cable to the elongate tubing, or sections of tubing before the tubing is placed downhole. The cable may be secured by clamps designed to withstand pressure downhole.
In accordance with a further aspect of the present invention, there is provided a well comprising a borehole, elongate tubing placed within the borehole so as to form an annulus extending along the length of the borehole, and a cable placed within the annulus, the cable having a cross-section, perpendicular to the length of the borehole, which has a major and a minor dimension.
The cable may have the preferred features as set out above.
The cable preferably adjoins the elongate tubing, such that the minor dimension of the cross-section runs along part of the borehole radius.
Desirably the cable is secured within the annulus by introducing cement, or thixotropic fluid, into the annulus.
The substantially elongate cross-section of the cable ensures that by appropriate placing of the cable, the distance between the cable and the outer wall of the borehole is maximised. This improves the likelihood of successful cementing of cable into the borehole.